1. Field of the Invention
The present invention relates to an optical cross connect apparatus and an optical transmission system based on an optical cross connect apparatus, and more particularly to an optical cross connect apparatus that is capable of changing an optical level path and an optical transmission system based on such an optical cross connect apparatus.
2. Description of Related Art
In recent years, there is a demand for an increase in the transmission network speed and capacity due to rapidly increased data traffic. To provide an increase in the transmission network speed and capacity, an optical transmission apparatus (WDM apparatus) is employed for a network. The optical transmission apparatus (WDM apparatus) subjects an optical signal to wavelength division multiplexing (hereinafter referred to as WDM). In WDM, a 10-Gbps high-speed optical signal is subjected to wavelength multiplexing so that an optical signal can be transmitted over a maximum distance of 80 km. Path switching for a 10-Gbps high-speed signal cannot be accomplished with an electrical signal. Such path switching must be performed after serial/parallel conversion is effected to reduce the speed to approximately 150 Mbps. When, on the other hand, a 10-Gbps high-speed optical signal is used, switching takes approximately 1 ms, but switching can be effected without having to reduce the signaling speed. Further, the use of a 10-Gbps high-speed optical signal for switching is at an advantage in that various signal formats can be handled.
The optical cross connect apparatus (hereinafter referred to as the OXC apparatus) is connected to a WDM net, which is formed by the aforementioned WDM apparatus, to perform path switching during the use of an optical signal. In general, the OXC apparatus is not only connected to a WDM net but also connected to a client apparatus, which is a large-scale router or digital cross connect apparatus. A transmission path block between two OXC apparatuses that establish communication between two client apparatuses is referred to as a path. This path may include a third OXC apparatus. The third OXC apparatus simply allows an optical signal to pass as viewed from the aforementioned client apparatuses. It seems that the connections of optical switches in the OXC apparatuses are fixed. However, a path may be formed between the third OXC apparatus and a fourth OXC apparatus. A transmission block between the OXC apparatuses is referred to as a span.
When switching is effected with an electrical signal and without using an OXC apparatus, a transmission apparatus corresponding to the aforementioned third OXC apparatus often connects a low-speed electrical signal, which corresponds to a passing optical signal, directly between transmission apparatuses. In such an instance, the employed network has lost flexibility.
To provide communication redundancy between client apparatuses, a scheme for setting an auxiliary path between the client apparatuses is considered. However, optical level path switching is not defined by the ITU-T or IETF, which is a standardization organization. Under the present circumstances, therefore, OXC apparatuses that are manufactured by different makers in compliance with different specifications cannot be connected face-to-face.
When the SDH/SONET or other protocol based on a telephone line is used, line switching can be quickly performed with a control channel that is superposed over a main signal. A line switching system disclosed by Japanese Laid-Open Publication No. 2004-104186 is for use in a system for establishing communication by using a protocol having no line switching function. This line switching system causes a line switching apparatus to perform line switching when a fault in the main system is detected, transmit a switching request signal to an opposing line switching apparatus, and ensure that the same line is selected. Japanese Laid-Open Publication No. 2004-104186 assumes that a transmission apparatus to which a terminal is connected and a transmission apparatus to which a network is connected are defined in a fixed manner. Further, line switching performed by a line switching apparatus is based on an electrical signal. No optical switch is described in Japanese Laid-Open Publication No. 2004-104186.
An optical cross connect apparatus disclosed by Japanese Laid-Open Publication No. 2003-115798 uses an optical coupler to divide an optical signal into two branches and monitors for a fault in an auxiliary transmission path. However, Japanese Laid-Open Publication No. 2003-115798 does not describe an optical signal loss that is caused by branching.
“Optical Cross Connect Technology for All-Optical Network”, sheets 15, 19 and 22, which is written by Shoichiro Seno, describes the aforementioned background technology and an optical path protection technology based on a bridge and a selector.
Optical level path switching is neither defined by the ITU-T or IETF, which is a standardization organization, nor otherwise defined as a standard. Under the present circumstances, therefore, OXC apparatuses manufactured in compliance with different specifications cannot be connected face-to-face so that adequate interoperability is not provided. In a network in which no OXC apparatus is used, it is necessary to perform switching after high-speed signals are converted to low-speed signals, and network flexibility is not adequately provided.